This invention relates to an electrophotographic photoreceptor, particularly to one having a light-sensitive layer containing both a carrier generation material and a carrier transport material.
Electrophotographic photoreceptors have been known extensively that have light-sensitive layers chiefly composed of inorganic photoconductors such as selenium, zinc oxide, cadmium sulfide and silicon. However, these photoreceptors have not necessarily been satisfactory in such characteristics as heat stability and durability. They also have had problems in connection with handling in manufacture.
On the other hand, photoreceptors having light-sensitive layers chiefly composed of organic photoconductive compounds have many advantages such as relative ease in manufacture, low cost and easy handling. Further, they are usually more heat-resistant than photoreceptors using inorganic selenium as a photoconductive material. Among various organic photoconductive compounds used today, poly-N-vinylcarbazoe is best known and photoreceptors having light-sensitive layers chiefly composed of charge transfer complexes formed of this compound and Lewis acids such as 2,4,7-trinitro-9-fluorenone are already in commercial use.
A proposal has also been made that different materials be used to fulfill the two principal functions of photoconductors, i.e., carrier generation and carrier transport, and photoreceptors of such a "functionally separated" type are known to incorporate carrier generation and transport materials either in superposed layers or in a single layer. As an extension of this approach, a photoreceptor having a light-sensitive layer composed of a carrier generation layer in the form of a thin amorphous selenium layer and a carrier transport layer containing poly-N-vinylcarbazole as a chief component is already in commercial use.
However, poly-N-vinylcarbazole is inflexible and its film is so rigid and brittle that it will easily crack or separate from the substrate. Thus, photoreceptors using this compound as a photoconductive material do not have high endurance. If one attempts to solve this problem by adding plasticizers, high residual potential will develop in electrophotographic processing and during cyclic use, the residual potential builds up to cause increased fogging until the copy image is substantially impaired.
Low-molecular weight organic photoconductive compounds usually do not have a film forming ability and hence are used in combination with suitable binders. This practice is preferred in that the physical properties or sensitivite characteristics of the photoconductive film can be controlled to some extent by properly selecting such factors as the type of binder used and its compositional ratio. However, the types of organic photoconductive compounds that are highly miscible with binders are limited and there are not many binders available that can be used to construct light-sensitive layers in photoreceptors, particularly in electrophotographic photoreceptors. For example, the 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole described in U.S. Pat. No. 3,189,447 has only low miscibility with binders such as polyesters and polycarbonates that are commonly used in the light-sensitive layers of electrophotographic photoreceptors. If this compound is used in the amount necessary to provide desired electrophotographic characteristics, oxadiazole will start to crystallize at 50.degree. C. and above to impair the electrophotographic characteristics such as charge retention and sensitivity.
On the other hand, the diarylalkane derivatives described in U.S. Pat. No. 3,820,989 are comparatively satisfactory in terms of miscibility with binders. However, they are not highly lightfast and if they are used in the light-sensitive layer of a photoreceptor that is to be subjected to cyclic transfer electrophotography where charging and exposure are repeatedly performed, the sensitivity of the photoreceptor will gradually deteriorate.
U.S. Pat. No. 3,274,000 and JP-B-47-36428 (the term "JP-B" as used herein means an "examined Japanese patent publication") describe different types of phenothiazine derivatives but each of them has low sensitivity to light and its performance will deteriorate during cyclic use.
The stilbene compounds described in JP-A-58-65440 and JP-A-58-190953 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") are comparatively satisfactory in terms of charge retention and sensitivity but their endurance is not so good as to withstand cyclic use. The bisstilbene compounds described in JP-B-53 87227 do not have high solubility and their miscibility with binders is low.
Thus, none of the carrier transport materials discovered to date have characteristics that should be satisfied in order to fabricate practically acceptable electrophotographic photoreceptors.